Oh V., i feel for you, i really do. i have been struggleing w/ this since my little girl was the same age. if you can, do any-thing you can now to stop her. the results of my daughter doing it for 2 yrs. now is terrible. i feel so bad for her. her nails are just not pretty. she has even gotten to the point 2 times already where she has gotten her nails infected and we have to go to the dr. to get it drained and then some anti-biotics. not fun at all.
my husband too bites his nails, and i think that is where she picked it up from.
my suggestions that i have tried are
get her 2 squishy hand size ball, almost like a hacky sack type ball. 1 for the house and 1 for the car. and when you see her putting her finger's in her mouth, give her the ball and let her keep her hands/finger's busy w/ that.
if you can at the time she is biting her nails, maybe suggest she come play playdoh? or have her come talk to you, or come BUSY her hands in any other way.
i notice my daughter doing it while watching t.v. and while driving in the car. i don't think that it nessassarily means that our kids are nervous kids, they just need to busy their hands in another way.
i have also tried that yucky tasting nail polish, i got it from walgreens. that worked for a bit, but then she caught on.
so i hope you can get her to stop. it is a horrible habit to have and to get someone so little to not do it. good luck.
C.